Arrangement for optimizing servo controller power in two-dimensional flexure mems storage devices

ABSTRACT

An arrangement for optimizing a servo controller power required in the operation of two-dimensional flexure (Microelectronic Memory Storage) MEMS devices.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of optimizing a servocontroller power required in the operation of two-dimensional flexure(Microelectronic Memory Storage) MEMS devices. Furthermore, theinvention is directed to an arrangement for optimizing servo controllerpower in a two-dimensional flexure MEMS storage device through autilization of the inventive method.

The technology of employing microelectronic memory storage devices(MEMS) is widely employed in the manufacture and commercial andtechnical applications of essentially low-cost, high-density memorystorage devices. In essence, pursuant to an example of the technology,which is currently in further stages of advanced development, resides inthe employment of flexure-based MEMS devices (F-MEMS), which possesses apotential of enabling a memory storage of information of 1terabits/square inch areal density. In effect, information is stored inthe MEMS device through the heating up of a small cantilever probe andthen producing rows of 40 nm indentations or pits on apolymethylmethacrylate (PMMA) layer of a thickness of 50 nm. Moreover,the very same cantilever probe can be readily employed for the readingback of the information by a method of sensing the presence or,alternatively, the absence of the pits or indentations, which have beenformed. Hereby, the polymethylmethacrylate (PMMA) substrate or layer isarranged so as to be mounted on a scanner platform, the latter of which,in turn, is supported by a number of flexures or flexural supports.Basically, two actuators are employed to move a scanner comprising areading/writing sensor about the surface of the PMMA layer forretrieving the information thereon or for writing information, into theright position along, respectively, the X and Y directions.

SUMMARY OF THE INVENTION

Pursuant to the present invention, various methods can be employed forthe purpose of minimizing the energy or power which is required to befurnished for powering the servo control for a flexure-based MEMSstorage device (F-MEMS), which possesses two degrees (2°) of freedom, ineffect, in the X and Y directions. Generally, in a normal manner, datais commonly organized in rows and columns, and then the data, such as,the indentations or pits that are formed on the PMMA layer, are accessedthrough a movement of a scanner comprising a reading and writing sensorfrom an initial idle home position in displacements along theappropriate X and Y scanning directions. Thus, while the reading andwriting sensor is located in a static or idle at home position, theflexural supports or flexures, on which the scanner platform issupported, are at rest and no energy or power is consumed in order tomaintain the sensor at that particular at-home location. However, inorder to deflect the flexures and move the reading and writing sensor toother locations on the surface of the PMMA layer away from the at homeposition in, respectively, X and/or Y scanning directions, power must besupplied to the flexural supports in order to deflect the latter tofacilitate movements of the sensor. This power requirement risesproportionally to the square of the distance in which the flexuralsupports or flexures are deflected. Although various designs may beemployed in order to minimize the amount of power, which is required forthe servo, which activates the displacement of the flexural supports;this requires different concepts in the utilization thereof. Forexample, data can be organized in various zones on the PMMA layer,whereby data, which is employed in a more frequent manner, is locatedcloser about the at-home position of the reading and writing sensor.This enables such frequently employed data to be accessed more rapidlyand at an expenditure of a much lower energy level or power requirement.For instance, in order to carry out random scanning movements seekingdata on the PMMA layer, by the reading and writing sensor, the requiredlengthier movement is initially implemented, and the movement along theother shorter access length is delayed an appropriate amount of time,whereby both scanning movements seeking the data are completed atgenerally the same instant in time. This, in essence, will provide aconsiderable reduction in expended energy, for example, by thesynchronized timing of two-micron movements, and a considerably greateramount of energy or power may be saved for even lengthier movements ofthe sensor.

Accordingly, it is an object of the present invention to provide a novelmethod of optimizing servo control power expenditures in atwo-dimensional flexure MEMS storage device.

Another object of the present invention is to provide a method of thetype described wherein a read/write sensor is movable about the surfaceof an X-Y scanner platform, which utilizes flexural supports for thescanner platform in order to be able to move the sensor in the desiredX-Y-directions at a minimal expenditure of servo control power.

Another object of the present invention resides in the provision of anarrangement for the optimizing of expenditures in servo control power intwo-dimensional flexure MEMS storage devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference may now be made to the following detailed description of thepreferred embodiment of the invention, taken in conjunction with theaccompanying drawings; in which:

FIG. 1 illustrates, generally diagrammatically components of aflexure-based MEMS storage device equipped with an X-Y scanner for aread/write sensor;

FIG. 2 illustrates an enlarged scanner probe detail from the encircledportion A in FIG. 1;

FIGS. 3 and 4 illustrate, respectively, side and top plan views in adiagrammatic representation of an optical position sensor employed in atest configuration on an X-Y scanner platform pursuant to the invention;

FIG. 5 illustrates, generally diagrammatically, an enlarged view of adetail in the encircled portion B of FIG. 4;

FIG. 6 illustrates, diagrammatically, data seek and scan trajectoriesfor the X-Y scanner of the present invention;

FIG. 7 illustrates a schematic illustration of a servo control systemfor implementing the X-Y motion of the scanner pursuant to theinvention;

FIG. 8 illustrates, diagrammatically, zonal data storage capabilities,illustrated diagrammatically pursuant to the invention;

FIGS. 9 a and 9 b illustrate respectively energy saving charts in thestorage of the data in a main region proximate the at-home position ofthe read/write sensor;

FIGS. 10 a, 10 b and 10 c illustrate respectively plotted time versusdistance functions of the X and Y scanner for the displacement of thesensor and the retrieving and/or writing of data; and

FIGS. 11 a and 11 b illustrate, respectively graphical representation ofthe energy savings of synchronized scanning seeks implemented by theF-MEMS apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the invention and particularly FIGS. 1 and 2of the drawings, there is generally diagrammatically illustrated acantilever probe arrangement 10 for reading back information in regardto an MEMS storage device 12 through the sensing of either the presenceor absence of indentations or pits 14, which are formed in the uppersurface 16 of a substrate which is constituted of a thickpolymethylmethacrylate (PMMA) layer 18. The PMMA substrate layer 18 ismounted in a lay flat mode on a scanner platform 20, the latter of whichis supported by a number of flexural members or flexures 22. In thisinstance, the flexural members 22 are illustrated as being constitutedof simple spring elements arranged on a stationary base 24, and haveeach one end 26 attached to the platform 20 and opposite ends 28attached to stationary base 24, whereby the scanner platform is able tobe subjected to a displacement in both the X and Y directions of thelatter. Connected to the scanner platform 20 is an X position sensor 30and a Y position sensor 32, which measure the displacement of thescanner platform, and wherein the displacement is imparted to thescanner platform 20 by means of an X actuator 34 and a Y actuator 36.

Contacting the upper surface 18 of the substrate which is comprised ofthe PMMA layer 18, is a scanning sensor 40 utilizing read and writeelectronics and which is adapted to, respectively, sense the presence orabsence of the indentations or pits 14 representative of data which havebeen formed in the surface 18 of the PMMA layer 20, such indentations orpits not being illustrated in FIG. 1 for purposes of clarity. The datastorage media for the MEMS device is arranged in the X-Y scanning plane,extending along the upper surface 18 of the PMMA layer 20.

As illustrated in the detail of FIG. 2 of the drawing, representing theenlarged encircled segment A from FIG. 1, there is illustrated a probe42 from scanning sensor 40, showing a resistive heater 44, and thedata-providing indentations or pits 14 which are formed in the uppersurface 18 of the polymer layer 20; i.e., the PMMA layer located on thescanner platform 20. The probe 42 is adapted to contact or sense theindentations 14, and in this instance showing the movement thereof alongthe X-directions of the layer 20.

As illustrated in FIGS. 3 and 4 of the drawings, the position ofscanners 50, 52 relative to a respective read and write (R/W) probe orsensor 54, 56 movable along the edges 58, 60 of scanner platform 20 is anecessary function in order to achieve reliable data storage functionsand information. Consequently, in order to accurately sense the positionto enable servo controlling, according to FIGS. 3 and 4, X and Y opticalsensors are employed, as shown in FIG. 5, above and below the scannerplatform, whereby a light beam is transmitted through an optical fiberin order to direct a light against the movable edge of the MEMS. Thelight beam passing through the optical fiber is then reflected throughan angle of approximately 90° utilizing a miniature prism structure 64,the latter of which, for example, may be 1 millimeter in size.

Furthermore, the light beam, which passes over a movable edge, is thencaptured by a second prism, deflected a further 90° and transmitted backto the scanner platform or portion of the sensor electronics.Consequently, the amount of light which is received in proportion to thelight which is transmitted forms the basis of a voltage output of theedge sensor 54, 56, and the voltage is then linearly correlated with thelocation of the platform edge.

Basically, as represented, the read/write operations of the sensorrequire two widely different position control capabilities, asillustrated in FIG. 6 of the drawings, in a diagrammatic planrepresentation. In that instance, a two-dimensional random seek andtrack follow scan is employed wherein scanning sensor 40 with no controlforce of light thereto, in effect, in a relax mode, is initially isaddressed in the home position, located in FIG. 6 and designated bypoint “A”. In order to implement an active operation in the movement ofthe sensor or read and write electronic component, which necessitates amovement thereof away from the home position, and when access to a datablock (i.e., indentations 14), which is present on the PMMA layer 20 isrequired, the scanning sensor 40 must be moved from location “A” to afurther location at point “B” on the surface 18 along two dimensions, ineffect, along an X trajectory and a Y trajectory. Nominally, the X seekor movement in the X direction is identical for all data blocks, whereasthe Y scan or seek is effected in a random manner. Once the location “B”is within reach, the scanning sensor 40 must change the velocity factorthereof in order to move along a path in order to move towards alocation at point “C”, and read or write data along the path ofmovement. For length data records, the scanning sensor must be able toreach the end of a path of travel along a +X access, then reversedirection and execute a reverse movement scan along the (−X) accesstowards a location at point “D”.

The scanner developed for this application has the freedom to moveindependently or selectively along X and Y Cartesian coordinates. Thus,two distinct position sensors and two feedback servo loops controllingtwo electromagnetic actuators 34, 36, schematically shown in FIG. 1, areemployed to implement the disclosed invention. It is noted that in FIG.1, the freedom to move along X-Y coordinates is in reality provided by acomplex system of flexures (details not shown), but schematicallyrepresented by a single “spring” element 22 for each degree of freedomof motion.

A proportional-integral-derivative (PID) servo controller is used inthis MEMS storage device. The characteristic PID controller transferfunction, for example in analog form, is represented by the followingexpression:

Controller (Output/Input)=(K _(p) +K _(D) S+K ₁ /S)  [1]

where gains K_(p), K_(D) and K₁ are proportional, derivative andintegral gains, and “S” is the Laplace transform operator. Theparametization process to compute the gains is well known in thetechnology. A control system designer would thus use a dynamic model ofthe scanner, and would derive the gain values in order to achieve an“optimum” design.

The servo system is required to perform three critical tasks. First, itmust move the scanner along the X and Y coordinates to the vicinity of atarget track (Location B in FIG. 6) within a minimum time using avelocity servo in a seek mode. To facilitate a robust and reliable seekto a target track, a desired velocity profile is typically stored in amemory and a velocity servo (in contrast to a position servo) isemployed to reach the vicinity of a target path. Next, the controlsystem must position the scanning sensor 40 on the track center line(TCL) of a target path using the Y-direction servo with minimumsettle-out time using a position controller of the type shown inEquation (1), with K₁ normally set to 0. Finally, the Y-servo systementers the track-follow mode with the Y-servo having aproportional-integral-derivative type (PID) position controller and theX-servo entering a scan mode desiring a fixed, predetermined scanvelocity (by either using a position servo or a velocity servo). Thisoperation is referred to as a track-follow-scan mode to emphasize thefact that the Y-servo is maintaining the storage media along a TCL asthe X-servo persistently maintains a predetermined scan velocity. Bothservos need to maintain precision against disturbances, such as unknownhysteresis effects and vibration.

The complete servo architecture to achieve this operation, as well asthe X-Y seek, is shown in FIG. 7. It is to be noted that for completelydecoupled dynamics of a scanning sensor along the X and Y coordinates,the servo system can be selected to have identical building blocks, butdifferent controllers (positions verses velocity) may be switched in andout of the servo loop at various phases of the scanner motion. Theposition information is generated by the previously mentioned opticaledge sensors 54, 56 and converted to a stream of digital numbers (at 5kHz in this example) by an analog-to-digital converter (ADC) 68. Adigital controller for each axis consists of a position controller block70; velocity estimator blocks 72, 74; velocity controller blocks 76, 78;reference trajectory blocks 80, 82; and post filter banks 84, 86. Underthe supervision of a microprocessor (not shown), the functions providedby the blocks are activated appropriately. The computed control outputin digital form is sent to a digital-to-analog converter (DAC) 88, 90 ata rate equal to or different from, the input sampling rate. The analogsignal generated by the DAC drives a current amplifier 92, 94, which inturn energizes the actuator 34, 36 of the scanner 40.

In compact flash memory, it takes the same amount of energy tosequentially read/write a block of data, independently of its locationand data is stored in the next available slot from the last write. Thisis not the case with a flexure-based MEMS storage device. It takes thesame amount of energy to read or write a bit but it takes additionalenergy to overcome the force of the flexure to access data further outfrom the home position. This, in turn, has a big impact on how data isstored in power sensitive applications. Herein, it is proposed that dataare stored in zones, depending on the data type and the operating modeof the device (FIG. 8). Zone 1 clusters around the home position for thescanning sensor 40, and it is reserved for frequently used data or datathat need to be retrieved in the shortest amount of time. Zone 2 is forincoming new data, whereas the oldest data reside in Zone 3. Datamigrate from the inner zone to the outer zone when the hosted device,i.e., notebook computer, is recharging and the F-MEMS 12 is idle. InZones 1 and 2, data grows outwardly from the home position (indicated bythe arrows). In Zone 3, data grows inwardly toward the home position,and the oldest data are saved at the furthest spot from the homeposition. The boundaries between zones can be shifted depending on auser's application.

FIGS. 9A and 9B illustrate the difference in accessing two identicalblocks of data in two different zones. Each block of sequential data isspread over 5 horizontal scan lines of 10 micron in length. It takes105.09 Joules to access the data block in Zone 2 or 3, located at 5microns from the home position (FIG. 6 a) and 37.88 Joules for accessingan identical data block clustered around the home position (Zone 1).This is an energy saving of 64%. This number becomes even higher if itis compared to a data block that is located further out from the homeposition, such as 100 microns away.

With an F-MEMS, energy can be further conserved by applying someintelligence to random seeks. First, needed to know are the seek timesTx, Ty as a function of distances X, Y, respectively. Tx and Ty can beexperimentally measured or estimated from the equations below:

Tx=Tx0+(X−X0)/Velocity-X

Ty=Ty0+(Y−Y0)/Velocity-Y

Wherein Tx0 is the time required to move the minimum distance X0,including acceleration to Velocity-X and slow down to 0. Velocity-X isthe seek velocity for the X-axis. Similarly, Ty0 is the time needed tomove Y0 distance and Velocity-Y is the seek velocity for the Y-axis. Txand Ty are plotted in FIGS. 10A and 10B, respectively, wherein FIG. 10Cshows the locus of X-Y seek timers. If Tx equals Ty, then this type ofseeks is represented by the diagonal line. The space above this line isfor cases that have longer Tx. Longer Ty cases are below the diagonalline.

Once Tx and Ty are, known, then it becomes possible to synchronize theissue of the X-Y seek commands such that both commands are completed atthe same time. For example, if Tx and Ty are the same, then both seeksare issued at the same time. If Tx is N milliseconds longer than TY,then X seek command is executed first, then Y seek will be delayed by Nmilliseconds.

FIG. 11A shows experimental data of a random seek case where X moves by6 micron, Y moves by 2 microns and both seek commands are issued at thesame time. Y has a shorter distance to move so it arrives much earlierthan X and power is needed to fight the flexure stiffness to keep the Yat 2 microns. Needed are 1.472 Joules for this two-dimensional seek andscan across one line of data. In FIG. 11B, X-Y seeks are synchronized sothat both arrive at the same time and no Y power is wasted waiting for Xto arrive. Here, the same two-dimensional seek and one line scan consumeonly 1.257 Joules, an energy saving of 14.6%. This saving will be morewhen X-distance is much larger than Y-distance.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the scope and spirit ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. An arrangement for optimizing servo controlled power in the operationof two-dimensional flexure microelectronic memory storage devices,comprising: a stationary base member; a movable scanner platform havingmemory storage media located on an upper surface of said scannerplatform; flexural supports for said scanner platform being positionedon said stationary base member facilitating displacement of said movablescanner platform in x- and y-directions relative to said stationary basemember; actuators connected to said movable scanner platform and beingresponsive to the operation of servo controllers for imparting said x-and y-directional displacements thereto, including x- and y-positionsensors for determining the extent of displacement of said scanningplatform; and a scanning structure movable along x- and y-coordinatesabout the surface of said scanner platform containing saidmicroelectronic memory storage media to the vicinity of a target trackwithin a period of time minimizing power output required for said servocontrollers.
 2. An arrangement as claimed in claim 1, wherein said servocontrollers reduce power requirements for said actuators by a zonal datastorage array on said movable scanner platform.
 3. An arrangement asclaimed in claim 2, wherein said zonal data storage array on saidscanner platform comprises clustering of frequently employed data abouta home position of said scanning structure.
 4. An arrangement as claimedin claim 3, wherein said servo controllers actuate said actuators for x-and y-displacements so as to synchronize random seeks in themicroelectronic memory storage devices so as to necessitate minimumpower requirements for random data seeks on said scanner platformsurface.
 5. An arrangement as claimed in claim 3, wherein said data isarranged on said scanner platform so as to propagate outwardly inpredetermined zones and inwardly towards said home position in furtherzones.
 6. An arrangement as claimed in claim 1, wherein said scanningstructure comprises an electronic read and write device displaceableabout the data-containing surface of said movable scanner platform. 7.An arrangement as claimed in claim 1, wherein said scanning structurecomprises a scanner probe positioned at the peripheral edge of saidmovable scanner platform.
 8. An arrangement as claimed in claim 7,wherein said scanning structure comprises prisms forming said scannerprobe on opposite sides of the scanner platform edge.
 9. An arrangementas claimed in claim 1, wherein each said x- and y-servo controllercomprises, respectively, operative system velocity estimators, velocitycontrollers and position controllers in operative interconnections withsaid x- and y-actuators and sensor electronics determinative of thedisplacement of said scanning structure on the surface of said scannerplatform.
 10. An arrangement as claimed in claim 1, wherein said data isprovided on the surface of said scanner platform as an array of surfaceindentations or pits.
 11. An arrangement as claimed in claim 10, whereinsaid indentations or pits are formed in a polymer layer located on theupper surface of said scanner platform; and said scanning structureincludes a resistive heater probe for contacting said indentations andpits to derive data therefrom. 12-22. (canceled)